Update: I've tried to increase the allowance and get a long list of errors.

Could not set allowance: could not form any contracts:
xx.xx.xx.xx:9982: wallet has coins spent in incomplete transactions - not enough remaining coins
xx.xx.xx.xx:9982: wallet has coins spent in incomplete transactions - not enough remaining coins
...

What does this mean? I've enough unlocked coins and also the renter has unspent funds?

We are planning on adding bandwidth limits to Sia soon, so that we don't need to be consuming 100% of your memory. In the meantime, there are tools for most systems that will allow you to throttle the amount of network usage a single process is consuming.

A few months ago we were not anywhere near fast enough to need to be worried about adding ratelimits, the fact that people are asking for them shows how far we've come. We'll be adding them soon.

Hey. I think it will be good enough to make sure you load 'siad' after you mount the storage. I hadn't considered the case you'd be mounting the storage folders after starting 'siad', but I'm pretty sure it will get confused if you do that.

It's probably something we can add support for in the coming months, but for now your best solution is to make sure everything is mounted before 'siad' starts running.

In the latest version of Sia, you can fix this using the Terminal view. Use the command wallet init --force. This will delete your existing wallet and generate a new seed phrase. You should then be able to unlock the wallet with the new seed.

What version?
The very first time you unlock it will take a long time if you are using an external HDD. Perhaps several hours :(
If you are using v1.2.0 or higher, after your first unlock it'll be a lot faster.

You can use Sia on an external HDD, but it's going to be very slow. We recommend running Sia on an SSD, and then if you are a host you can add external HDDs as storage folders and this will not be too slow.

Currently the best way to do cold storage is to run Sia on an offline machine. Since it is offline, it will not be able to sync. That's okay though, because it will still be able to create addresses in its wallet. You can send money to those addressses, and then confirmed that it worked by looking at the explorer.